Radiofrequency (RF) and cryogenic ablation procedures are well recognized treatments for vascular and cardiac diseases such as atrial fibrillation. The application of either RF or cryogenic treatment is usually based on the preference of the surgeon or the specific tissue to be treated. In either RF or cryogenic ablation, however, the location and quality of the lesion produced is a primary concern. The clinical success of cardiac tissue ablation to treat arrhythmias depends on efficacy and safety of the application of the selected energy. Many factors influence lesion size such as tissue-electrode contact force, ablation energy level, and cooling factors, that is, blood flow rate, tissue perfusion, and the duration of energy delivery. In addition, there are other factors that can limit deep lesion formation, such as early impedance rise that prevents continued energy delivery. Predicting and assessing lesion size and quality is important to the success of the ablation, but it has been difficult to achieve.
Current methods to identify a lesion's location and assess its quality include coupling a plurality of electrodes to the distal end of a medical device proximate a tissue to be treated, applying a voltage, and measuring impedance across the electrodes with the tissue to be treated completing the circuit. Electrical impedance is defined as the total opposition to alternating current by an electric circuit, equal to the square root of the sum of the squares of the resistance and reactance of the circuit and usually expressed in ohms. In general, the impedance decreases as cell membranes become ruptured and cellular fluids are released into the extracellular space in the regions of treated tissue. These treated regions then become necrotic. As such, impedance may be used to identify particular areas which have been treated and those that have not. It also should be noted that when sufficiently high voltage is applied to tissues, cells may undergo irreversible electroporation which creates permanent pores in the treated cell membranes. This process also releases fluids into the extracellular spaces and leads to tissue necrosis as do the RF and cryogenic therapies.
One drawback to impedance tomography is its lack of direct feedback to evaluate whether a lesion was successfully created to the desired transmurality, quality, or continuity. In particular, impedance measurements provide binary data regarding a particular lesion; either the tissue is viable or necrotic. Impedance measurements alone, however, do not provide real-time assessment of whether a cryogenic or RF lesion was successfully created to a desired lesion depth, in part, because different tissue levels have different impedances.
Tissue ablation technology often utilizes catheter tip temperature monitoring with feedback control to titrate energy delivery. The major limitation of this approach is that the catheter tip temperature and tissue temperatures are not the same. The catheter tip temperature is consistently lower than tissue temperature. The difference is variable and is dependent on the force of the catheter tissue contact that determines impedance as well as cooling of the catheter tip.
In view of the above, it would be desirable to provide improved methods of assessing tissue contact, lesion quality and depth, and other characteristics of cryogenically and/or RF treated tissue to determine the efficacy and resulting characteristics of the treatment.